Actually, most leather used unless it is super high end came from cows that died due to natural causes and were sent to a rendering plant, or it is the byproduct of cow slaughter. Very rarely is a cow slaughtered primarily for its hide, that would be very wasteful.
If you went by that same philosophy vegans could not use makeup or shampoo, hairspray,clothing(after all how do you think they test the dyes used to make those cute prints to be sure they wont irritate your skin) bug spray, medicine or anything else for that matter. Somewhere along the lines allll of it has been tested on animals (unless of course it is explicitly marketed as not tested on animals) and is a cause of cruelty to animals, much more than taking an already dead cows hide.
So really all vegans are hypocrites unless they don't take medicine and spring to spend the extra bucks on he no animal tested products.